Medical devices deliver electrical stimulation in order treat a variety of ailments or symptoms of patients, such as pain, epilepsy, and movement disorders. Often, the medical devices used to treat such ailments or symptoms are implantable. Further, whether implanted or not, the medical devices often deliver stimulation to tissue via one or more conductors within a lead, which may include one or more lead extensions, and the electrodes associated with these conductors that are carried by the lead.
Over the life of the medical device, the impedance presented to the medical device by the conductors, electrodes, and tissue proximate to the electrodes, e.g., the impedance of the conductors, electrodes, and tissue, may vary. This variation in presented impedance may be due to a variety of factors, such as lead material degradation, changes in patient hydration, and changes in the type of tissue proximate to the electrodes, which may be due to, for example, movement of the electrodes relative to the tissue intended to be stimulated, or growth of tissue at the stimulation site. When the patient is active, e.g., moving and/or changing postures, the electrodes may move relative to the tissue intended to be stimulated.
Medical devices that deliver electrical stimulation generally fall into one of two categories. Constant voltage devices include a voltage source, such as a voltage regulator, that outputs the stimulation at a specified voltage amplitude, while constant current devices include a current source, such as a current mirror, that outputs the stimulation at a specified current amplitude. These voltage and current sources hold their stimulation at a constant voltage or current amplitude, respectively. However, due to changes in the presented impedance over time, the current output by a constant voltage device, and the voltage output by a constant current device, will vary.
Clinicians may prefer either a constant voltage or constant current device for any of a variety reasons. For example, some clinicians prefer one of these types of devices for a particular therapy. Others may prefer one of these types of devices due to familiarity, or for other idiosyncratic reasons.